The present invention relates to a method of channel coding a parameter, of which the values are correlated with each other to different extents.
The breakneck pace of technical development in the field of mobile communications has, in recent years, led to the development of what are known as Adaptive Multirate (AMR) Narrowband and Wideband speech codecs which are designed to be used in future mobile radio standards such as GSM/EDGE and UMTS. An AMR codec makes it possible to adaptively adjust the entire data rate of the data to be transmitted as well as to subdivide the data rate into source and channel coding depending on channel status and network conditions (system load). The main objectives of these types of AMR speech codes are to achieve fixed network speech quality with a variety of channel conditions and to ensure optimum distribution of the channel capacity, taking account of specific network parameters.
An AMR has a number of codec modes (modes) which specify different rates of the coded speech bits. For a restricted transmission bandwidth, an AMR should be able to operate in high-rate modes under good channel conditions and/or in heavily loaded radio cells. Under bad channel conditions it should be dynamically switched into the lower-rate modes. As such, the best speech quality should be produced taking into account the changing channel conditions.
In the GSM/EDGE system, 2 bits (mode bits) that form what is known as the identifier are reserved in each speech frame of 20 ms (both in the full-rate and in the half-rate channel) and can specify up to 4 different modes of the AMR codec (CODEC_MODE—1, CODEC_MODE—2, CODEC_MODE—3 and CODEC_MODE—4). The 2 Bits id(1), id(0) of the identifier are coded via a block code. In this coding, a code word can be assigned to each parameter. The set of modes signaled by the identifier (maximum 4) is designated as the Active Codec Set of the basically 8 codec modes possible with AMR Narrowband. At least this Active Codec Set is known to the base station and the mobile station. Since the 2 mode bits together with the coded speech bits will be transferred within a channel, they also will be referred to as in-band data (see Table 1 and Table 2).
TABLE 1Channel coding of the parameter “identifier” inadaptive multi rate speech channel at full rate (TCH/AFS)(extract from GSM 05.03, Section 3.9)Encoded inIdentifierEncoded in-bandband data for(definedReceived in-data for SID andspeech framesin GSMband dataRATSCCH framesic(7), . . ,05.09)id(1), id(0)ic(15), . . , ic(0)ic (0)CODEC_MODE_100010100110000111100000000CODEC_MODE_201001111101011100010111010CODEC +10100010000110001101011101MODE_3CODEC_MODE_411111001011101010011100111
TABLE 2Channel coding of the parameter “identifier” inadaptive multi rate speech channel at half rate (TCH/AHS)(extract from GSM 05.03, Section 3.10)Encoded inIdentifierReceivedband data for(definedin-bandEncoded in-band dataspeech framesin GSMdata id(1),for SID and RATSCCHic(7), . . ,05.09)id(0)frames ic(15), . . , ic(0)ic (0)CODEC—0001010011000011110000MODE_1CODEC—0100111110101110001001MODE_2CODEC—1010001000011000110111MODE_3CODEC—1111100101110101001110MODE_4
On the receive side, this identifier, also referred to below as a parameter, is first decoded. Furthermore, depending on this decoding mode, and thereby on the current codec mode, the speech bits are determined. If the parameter and, thereby, the codec mode is, for example, incorrectly recognized because of a receive signal with a great deal of noise, the speech frame will be incorrectly decoded and the speech information can be lost.
The Initial codec mode for call setup or handover is defined as follows:
If all 4 codec modes are used, the initial codec mode is the codec mode of the Active Codec Set with the second lowest rate. Otherwise, the codec mode with the lowest rate. In each case, the codec mode changes from frame (k−1) to frame k only to one of its “adjacent” codec modes or remains in the same codec mode (timing correlation of the values of the parameters). A jump to a non-adjacent mode, such as from CODEC_MODE—1 to CODEC_MODE—3, is forbidden (see Table 3). In other words: The parameter “identifier” has a memory, or is correlated from frame to frame (inter-frame correlation or temporal correlation). The memory of the parameter “identifier” leads to a restricted change of the codec modes, or equivalently, of the code words (CW1, . . . , CW4) from frame (k−1) to frame k.
TABLE 3
The present invention is, accordingly, directed to a method for channel coding a temporally or spatially correlated parameter which allows reliable and low-overhead transmission of the parameter.